Electro-explosive devices fall into one of two basic groups. The first group is electro-thermally initiated devices which respond to relatively low electrical energies. The second group is electro-shock initiated devices which include exploding wire and foil designs requiring very high energy levels.
The shock initiated devices have the advantages of fast and repeatable function times. The shock initiated devices also exhibit a very high resistance to inadvertent initiation. However, high initiation energies and power levels are normally required which lead to larger and more expensive electrical firing systems.
The electro-thermally initiated group have not matched the inherent input safety characteristics or response time of the shock-initiated devices. Typical response times for the thermally-initiated devices range from about 5 microseconds to several milliseconds, while the shock-initiated electro-explosive devices respond in less than 1 microsecond. However, shock-initiated devices typically require larger and more expensive firing circuits for initiation because they use higher electrical voltages and dissipate to higher power levels.
In order to obtain environmental tolerance along with acceptable shelf-life, electro-explosive devices are usually designed with hermetically sealed housings with electrical feed-throughs. Additionally, thermally-initiated devices must be able to withstand reasonable, unintended currents without firing since relatively low energies are required to cause firing of the devices. Any current will produce some heating of the bridge wire and most designs of thermally-initiated devices have limited cabability to conduct this heat away from the thermally sensitive explosive. Prior art methods for preventing inadvertent firing of the thermally-initiated devices include using a large diameter bridge wire and thermally-conductive header dielectrics. This also tends to extend the explosive function time and is undesirable for many applications.
There are several examples of metal thin film bridges in the prior art. For example, U.S. Pat. No. 4,484,523 (Smith, et al.) issued on Nov. 27, 1984, discloses a semi-conductor detonator comprising a thick film bridge. However, a non-selectively deposited chromium-silicon film is used as the metal film layer.
U.S. Pat. No. 3,974,424 (Lee) issued on Aug. 10, 1976, discloses a variable resistance metal foil bridge element for electro-thermal devices. The resistance element is generally S-shaped and has two arcuate resistor portions which are joined by a connector portion. The effective resistance of the bridge element may be varied by changing the points at which the connection to the lead wires is made.
Blewer, R. S. and Wells, V. A., "Thick Tungsten Films in Multi-layer Conductor Systems: Properties and Deposition Techniques", 1984 Proceedings, First International IEEE VLSI Multi-level Interconnection Conference, New Orleans, La., 1984, discloses techniques for depositing thick films of tungsten onto metal and silicon surfaces. However, this publication does not disclose a method for fabricating a thin-film bridge device.
U.S. Pat. No. 3,669,022 (Dahn, et al.) issued on Jun. 13, 1972, discloses a thin-film bridging device which may be used as a fuse. The device includes a pair of conductive layers separated and joined to opposite faces of a thin insulating layer to thereby form a three-layer sandwich. The sides of each layer are coated by a bridge element of low-density, low-specific heat metals so as to short-circuit or bridge the conductive layers.
U.S. Pat. No. 3,682,096 (Ludke, et al.) issued on Aug. 8, 1972, discloses an electric detonator in which an incandescent bridge intended to set off a charge is formed on one side of a non-conductive carrier which is inserted into a conductive housing and which rests on its side opposite the bridge.
U.S. Pat. No. 4,586,435 (Bock) issued on May 6, 1986, discloses an electric detonator. In FIG. 4 of that patent, a fuse unit is shown which uses a tungsten filament. However, tungsten is not used as a bridging film element in this detonator, nor is it a supported thin-film structure.
U.S. Pat. No. 4,428,292 (Riggs) issued on Jan. 31, 1984, discloses a high-temperature exploding bridge wire detonator and explosive composition. The patent is primarily directed to an explosive composition, although it does disclose that the composition can be initiated by an exploding bridge wire or an electro-static discharge of sufficient energy.
Thus, there is a need in the art for metal film bridge devices which require less energy and which do not fire inadvertently as a result of electro-static discharge. In addition, there is a need in the art for metal film bridge devices which are simple to manufacture and which can be mass-produced.